In rotating electrical machines, such as motors or generators, the reliability of the insulating system is critically responsible for their operational integrity. The insulating system has the function of electrically insulating electrical conductors (wires, coils, bars) durably from one another and from the laminated stator core or the environment. Within high-voltage insulation, distinctions are made in insulation between partial conductors (partial conductor insulation), between the conductors or windings (conductor or winding insulation), and between conductor and ground potential in the slot and winding head region (main insulation). The thickness of the main insulation is adapted both to the nominal voltage of the machine and to the operational and fabrication conditions. The competitiveness of future plants for energy production, their distribution and utilization, is critically dependent on the materials employed and technologies applied for the insulation.
The fundamental problem with insulators loaded electrically in this way lies in the so-called partial discharge-induced erosion with formation of what are called “treeing” channels, which lead ultimately to the electrical breakdown of the insulator.
High-voltage and medium-voltage machines currently employ what are called impregnated laminar mica insulation systems. In these systems, the form-wound coils and conductors produced from the insulated partial conductors are enwound with mica tapes and impregnated with synthetic resin preferably in a vacuum pressure impregnation (VPI) process. The combination of impregnating resin and the carrier tape of the mica provides the present mechanical strength and also the required partial discharge resistance of the electrical insulation.
Mica paper is converted, in line with the requirements of the electrical industry, into a more stable mica tape. This is done by bonding the mica paper to a carrier material having a high mechanical strength, by means of an adhesive. A feature of the adhesive is preferably that it has a high strength at room temperature, in order to ensure the joint between mica and carrier, and passes into a liquid state at elevated temperatures (60° C.-150° C.). This allows it to be applied as an adhesive at elevated temperature in liquid form or in a mixture with a volatile solvent. After cooling has taken place or the solvent has been removed, the adhesive is present in a solid yet flexible form, and allows the mica tape to be applied, for example, around Roebel bars consisting of partial conductors and form-wound coils at room temperature, with the adhesive properties of the adhesive preventing delaminations of the mica paper from the carrier material. The resulting mica tape is wound in a plurality of plies around electrical conductors.
High-voltage and medium-voltage motors and generators employ laminar mica insulation systems. In these systems, the form-wound coils produced from the insulated partial conductors are enwound with mica tapes and impregnated with synthetic resin primarily in a vacuum pressure impregnation (VPI) process. In this case, mica is used in the form of mica paper, and as part of the impregnation the cavities located between the individual particles in the mica paper are filled with resin. The combination of impregnating resin and carrier material of the mica provides the mechanical strength of the insulation. The electrical strength comes about from the multiplicity of solid-solid interfaces in the mica used. The resulting layering of organic and inorganic materials forms microscopic interfaces whose resistance to partial discharge and thermal stresses is determined by the properties of the mica platelets. As a result of the complicated VPI process, even very small cavities in the insulation must be fully filled with resin, in order to minimize the number of internal gas-solid interfaces.
For the additional improvement of the resistance, the use of nanoparticulate fillers is described.
The combination of impregnating resin and the carrier tape of the mica provides the present mechanical strength and also the required partial discharge resistance of the electrical insulation.
As well as the VPI process, there is also the Resin Rich technology for producing and impregnating the mica tape, in other words the insulating tape and then, subsequently, the insulating system.
The main difference between these two technologies is the construction and manufacture of the actual insulating system of the coils. Whereas the VPI system is complete only after the impregnation and after the curing of the winding in a forced air oven, the leg of the Resin Rich coil, cured separately under temperature and pressure, constitutes a functioning and testable insulating system even before installation into the stator.
The VPI process operates with porous tapes, forming a solid and continuous insulating system under vacuum with subsequent exposure of the impregnating vessel to overpressure after curing in the forced air oven.
In contrast to this, the manufacture of Resin Rich coils is more complex, since each coil leg or winding bar has to be manufactured individually in specific baking presses, leading to a specific increase in the costs of the individual coil.
In this context, mica tapes are employed that are impregnated with a polymeric insulating substance which is present at what is called a B-stage. This means that the polymer, usually aromatic epoxy resins (BADGE, BFDGE, epoxidized phenol novolaks, epoxidized cresol novolaks, and anhydrides or amines as hardeners), is partially crosslinked and is thus in a tack-free state, but on further heating is able to melt again and be ultimately cured, so as to be brought into the final shape. Since the resin is introduced in an excess, it is able, during the final pressing operation, to flow into all cavities and voids, in order to attain the corresponding quality of insulation. Excess resin is pressed out of the system by the pressing operation.
From the literature it is known that the use of nanoparticulate fillers in polymeric insulating substances leads to significant improvements in the insulation in respect of the electrical longevity.
A disadvantage of the known systems, especially of those based on epoxy resins, is the rapid degradation of the polymeric matrix on exposure to partial discharge, here referred to as erosion. Implementing the polymer matrix with erosion-resistant nanoparticles (aluminum oxide, silicon dioxide) causes its exposure, brought about by incipient breakdown of the polymer, referred to as polymer degradation.